Two homologues of a novel protein identified in a Golgi proteomic analysis, GMx33 alpha & beta, are hypothesized to be Golgi matrix-like proteins that function in the regulated sorting and exit of molecules from the TGN. GMx33 fits the general description of Golgi matrix proteins in that it is detergent insoluble when associated with the Golgi membrane, contains a putative GRIP domain and coiled-coil regions, and is involved in several large protein complexes. Moreover, GMx33 is highly post-translationally modified in a dynamic manner that seems to control membrane association, and a yeast knock out strain, vps74delta, shows a vacuolar sorting defect in preliminary analyses. The role of GMx33 in sorting and exit from the Golgi will be tested using conventional mutagenesis to identify domains and sites that are essential for GMx33 a & b localization and function, RNA interference to reduce endogenous GMx33alpha & beta levels, and conventional biochemistry along with mass spectrometry to identify GMx33 modifications and binding partners. In addition, the vps74delta S. cerevisiae strain will be analyzed for sorting defects and complementation of the knock out will be achieved with various truncated and point mutated versions of GMx33 or Vps74p. The information gained through this study will extend our understanding of the dynamic properties the Golgi matrix and elucidate the reasons that different matrix molecules are required in cis- and trans-Golgi.